There is international concern about the amount of airborne toxic materials found in shooting ranges, particularly airborne lead. The Federal Government of West Germany has set the maximum permissible work-place concentration of lead dust at 0.1 mg of lead per cubic meter of air. During tests at German and non-German shooting ranges, equipped with various types of ventilation systems, concentrations of up to 9 mg of lead per cubic meter were measured over exposures of 4-6 hours when conventional, fully jacketed 9.times.19 mm Luger (parabellum) ammunition was used. This high airborne lead concentration is also found in some indoor shooting range using .22 caliber rimfire ammunition. The main problem is thought to come from the lead styphnate and barium nitrate used in the priming composition.
One attempt to solve the above problem for centerfire ammunition (which has a battery cup primer or Berdan primer placed in the center of the cartridge base) has been the use of "Sintox" primer developed by Dynamit Nobel of Troisdorf, West Germany which is thought to use an amorphous dinol initiating explosive mixed with zinc and titanium compounds rather than lead or barium compounds. However, that priming composition is not suitable for priming rimfire cartridges because it does not have the necessary sensitivity for use as a rimfire priming mix and because it does not have a small enough particle size to fit into the rim of the cartridge during the priming procedure. Also, "Sintox" primer is a centerfire primer and as is the case with most centerfire primer mixes, has less sensitivity than that required to function as a rimfire primer. Dinol-based priming mix having a smaller crystalline size than that of the "Sintox" primer was made and was tried by applicant's predecessors at Olin Corporation about 40 years ago, but that mix contained lead in forms of lead thiocyanate and lead peroxide and is thus not suitable as a lead-free primer mix. Thus, a need remains for a lead-free primer mix for .22 caliber rimfire cartridges.
Also of concern is the bullet. Caliber .22 bullets are conventionally made of lead which is sometimes plated with copper for aesthetic reasons. There is a concern that some of the airborne lead found in indoor shooting ranges may be caused by vaporization of the lead base of the bullet, frictional wear during passage of the bullet through the barrel or air and/or "dusting" (or minute fragmentation of the lead bullet) upon impact of the lead bullet with the target or backstop. Copper bullets, aluminum bullets and other non-lead bullets are known as a solution to the concern about airborne lead from the bullet.
A solution to this problem of needing a lead-free, non-toxic .22 caliber rimfire cartridge has been long sought after. Rimfires have been in existence for many, many years without such a primer having been found. It is well known that rimfire priming mixes must have a considerably greater sensitivity than that required by centerfire primers, so even if a low sensitivity lead-free centerfire primer mix is found, it is not obvious how to make a high sensitivity lead-free, non-corrosive, non-toxic rimfire priming mix. The phrase "Non-toxic" as used herein means consisting essentially of materials which are not heavy metals such as lead or barium and not known carcinogens or poisons, especially when vaporized, burnt or exploded as in the firing of an ammunition round. "Lead free" as used herein means having less than one per cent by weight lead. "Non-corrosive" as used herein means not containing halogens and thus producing any significant combustion products which are corrosive to iron or steel barrels. The problem is such that the U.S. Government's Army Research and Development Center issued a contract to Olin Corporation to study the feasibility of developing such a primer.
During this study, the present invention was made and is being recognized by both Olin and the Government as a major breakthrough in rimfire technology.
A solution to the above problem is achieved by the present invention which provides a rimfire cartridge which includes a shell case, a non-lead metallic bullet, a non-toxic rimfire priming composition, a non-toxic propellant powder and a nitrocellulose foil between the primer and the propellant. The preferred primer mix consists essentially of MnO.sub.2, tetrazene, dinol and glass. It is believed that manganese dioxide has never before been used as an oxidizer in ammunition primers, and especially not in rimfire cartridges. It was also believed that 13% nitrated nitrocellulose foils have never been used before in rimfire cartridges and it is further believed that there has never before been a non-toxic, non-corrosive, lead-free rimfire cartridge.